Recently, a silicon-based photomultiplier is actively developed, and also a weak light detection system using a scintillator and a photomultiplier (such as a radiation detection device adapted to detect an X-ray and the like) is further along in development. For example, the radiation detection device is utilized in computed tomography (CT) and the like and enables tomography of a patient, a luggage, or the like. Especially, as a photoelectric conversion element to be a detection unit of the photomultiplier, a SiPM formed by connecting, in series, an avalanche photodiode (APD) and a quench resistor has a high S/N ratio and a high dynamic range and achieves low-voltage drive. Such a radiation detection device utilizing the photoelectric conversion element detects current from the photoelectric conversion element, and acquires charge and voltage by integrating the current, and then the voltage (charge) is sampled and held to be subjected to AD conversion. An obtained digital signal is utilized to create a histogram and the like by signal processing.
On the other hand, in a radiation detection device of a photon counting system, an arrival rate of an X-ray entering an scintillator is estimated to be about 108 [cps], and a circuit that can measure data with high-speed and high-energy resolution by several hundreds of channels is demanded. Furthermore, a counting rate that can be detected by the above-described radiation detection device is varied by a recovery time of the photoelectric conversion element, conversion capacity of an AD converter, or the like. However, in order to shorten the recovery time of the photoelectric conversion element, there may be an exemplary method of reducing a time constant by setting a small value for a quench resistor of the photoelectric conversion. However, in the case where the value of the quench resistor is too small, quenching operation may not be able to be performed. Therefore, shortening the recovery time has a limit. Due to this, a so-called pile-up may occur when a phenomenon (hereinafter referred to as event”) in which a photon of radiation (or scintillation light converted by scintillator) enters the photoelectric conversion element occurs within the recovery time of the photoelectric conversion element.
Furthermore, a probability of event occurrence occurring in the radiation detection device is based on Poisson distribution. Therefore, in the case where an event interval is, for example, an interval shorter than an average arrival time 10 [ns] (equivalent to 108 [cps] described above) of the radiation, the probability of event occurrence is about six out of ten, and even in the case of 4 [ns] or less, the probability of event occurrence is about three out of ten. Therefore, the AD converter having high-speed AD conversion capacity is necessary in order to detect an event, but the high-speed AD converter consumes a large amount of electricity.